This application claims the priority of German Patent Document No. 101 43 561.4, filed 5 Sep. 2001, and PCT/DE02/03238, filed 3 Sep. 2002 the disclosure of which is expressly incorporated by reference herein, respectively.
The invention pertains to a process and a system for the location of emitters in the radar frequency range that is based on cross bearing (multi-platform triangulation) via airborne passive high frequency (RF) sensors.
Today, emitters in the radar frequency range are surveyed and located from a single airplane by means of continuous direction-finding and subsequent triangulation of the direction-finding beams. This method, however, has an insufficient accuracy in the case of emitters that are switched on briefly (with an emitting period of 15 seconds or less) and that are located at distances between the airplane and the emitter (greater) larger than approximately 25 km.
Processes have therefore been designed in which use is made of direction-finding beams from two airplanes that simultaneously intercept the same emitter. As a result of this, the transient triangulation base can be enlarged significantly so that it becomes possible even to locate emitters that are switched on only briefly.
This principle of position-finding by intersecting bearing lines is used, among other things, for locating flying objects via sensors that are stationed on the ground and that are spatially separated. Discussions and model calculations have been published in order to transfer this principle to flying platforms, whereby airborne emitters as well as ground emitters are surveyed and located with the help of multi-platform triangulation from several flying platforms. This principle was presented, for example, by K. Taubenberger and J. Ziegler in the lecture “Sensorfusion [sic] for Modern Fighter Aircraft” at the AGARD MSP Symposium “Advanced Architectures for Aerospace Mission Systems” in Istanbul, Turkey, Oct. 14-17, 1996, which was published in the AGARD Conference Proceedings AGARD CP-581 (obtainable via the Karlsruhe Technical Information Center, Eggenstein-Leopoldshafen).
Such multi-platform— triangulation can be used in the case that one or two emitters are in the area covered by the relevant sensors. In the case of many emitters in the covered area, however, this process of multi-platform-triangulation has the disadvantage that the number of geometrical intersection points is significantly higher than the number of real emitter positions.
In order to eliminate the very large number of “virtual” intersection points, the proposal is made in the lecture “MIDS Triangulation and De-ghosting of Intersection Points” by J. Ziegler and H. Sachsenhauser at the RTO SCI Symposium on “Sensor Data Fusion and Integration of the Human Element”, Ottawa, Canada, Sep. 14-17, 1998, which was published in RTO MP-12, that all the intersection points be processed over a (longer) period of time in order to be able to evaluate whether, in terms of their geometrical and kinematic characteristics, they then behave as real emitter targets. Several credibility criteria are described in this publication for evaluation purposes, and various scenarios are examined in a model calculation.
A disadvantage with the use of this method is that, in the case of the occurrence of a large number of emitters, the number of intersection points increases as a function of n2 with increasing the number n of emitter targets, and thus the requirements that are set for computer/processor performance increase very markedly. In the case that “emitter bearing lines” are only available transiently, on the other hand, the observation time is too short in many scenarios in order to be able to differentiate virtual intersection points reliably from a true target position. Thus in a scenario with several or many emmiters, which are switched on only briefly, the positions of the emitters cannot be determined unambiguously via this process, i.e., they can be only determined ambiguously.
Methods for signal processing and signal recognition have been disclosed in the publication “Radio communication and the waging of electronic warfare”, R. Grabau, 1986, Frank'sche Verlagshandlung, W. Keller & Co., Stuttgart, ISBN 3-440-05667-8, pages 337 through 343. In this connection, the correlation of identical or similar signals is carried out in order to classify radar devices. The individually registered signal components/parameters of emmisions of radiation, which are possibly identical in frequency or of the same type, are assigned to different radiation sources by means of signal processing. It is required to find one pulse sequence of a radar device amongst all the pulse sequences that are registered in one searching process; in order to do this, the temporal sequence of the individual registered impulses is stored in a memory, and the time differences between the various pulses are measured, and the entire impulse sequence is examined using the temporal sequences that are obtained. If one or more uniform impulse sequences arise, then these parameters are subjected to further examination, or they are used directly for classification purposes. In another method, several test sequences of pulses are generated and correlated with every registered pulse sequence, whereby the number of agreements between the parameter values decides whether a particular parameter value is used for the purpose of classification. In the case of pattern analysis, the characterizing features of the individual elements in the registered sources are separated into signals, and then they are structured. In the case of signal recognition, segmentation takes place in order to recognize signals and portions of signals that are associated with one another.
A device to passively locate a distant radiation source via receiving antennae is disclosed in U.S. Pat. No. 4,393,382, whereby these antennae are arrayed in a spatial triangle in order to obtain measurable the various electrical signals generated by the received radiation; the angle of arrival is determined from these time differences. In turn, the distance of the radiation source from the receiving antennae is derived therefrom.
An airborne surveillance method and system is disclosed in U.S. Pat. No. 4,910,526 in which allows an observer airplane to ascertain the position and change of position of a plurality of target airplanes in which form part of a threat scenarion. Use is made of various factors for determining the position of the target airplane such as the particular elevation at which the airplane is flying and the time differences of signals of secondary surveillance radars. The Kalman filtering technique is used to a produce an estimate of the position of the target airplane on the basis of earlier measurements and to get an estimate of the error magnitude of each measurement. Incorrect values are thereby carried along in the process until a signal value is recognized as being incorrect on thebasis that the estimated value deviates from this signal value. This method results in a very complex and hence costly process.
An object of the invention is to provide a process on the basis of multi-platform-based triangulation in which even radar emitters that are switched on only briefly, i.e., over time spans of 3 to 15 seconds, can be localized unambiguously and with high accuracy via flying platforms.
An additional object for the invention is that such a process can be implemented with limited computation costs and with a limited inter-aircraft transmission rate in an airplane system even in the case of a that the number of intercepted emitters is greater than two per frequency band of the participating sensors.
In contrast to the inter-linking of ground-supported sensors, the data transmission rate between today's airplanes is limited; for example, this data transmission rate lies in the UHF range of currently approximately 0.2 to 1 Kbit/s in the case of an airplane/airplane data link; ODIN that is implemented in the ECR-TOR currently permits only one UHF data transmission in the manual operation mode. Even on a medium midterm timescale, an effective data rate of only 3 to 10 Kbit/s can be expected with the installation of new UHF communication devices.
In accordance with the invention, the geometrical emitter beam data, such as the position of the airplane at the point in time of surveying the emitter and the azimuthal angle at which the emitter is surveyed, are exchanged between the participating platforms for the purpose of emitter localization. The angle of elevation, the azimuthal angle measurement error, and/or the point in time of intercepting the emitter are also optionally exchangeable.
Properties that describe the important characteristic features of the intercepted RF signal and its temporal profile are also transferred between the platforms.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.